1. Field of the Invention
The present invention relates generally to remote detection of gases and vapors at low concentration levels and over short path lengths. The present invention relates more specifically to a remote vapor detection system and method thereof, in which a laser is used to heat the background immediately adjacent the vapor or gas to be detected. Such heating increases the infrared radiation of such background which then serves as an infrared source for measuring the absorption characteristics of gases or vapor clouds between the background and a wavelength selective receiver.
2. Prior Art
U.S. Pat. No. 4,496,839 to Bernstein et al is directed to a system and method for remote detection and identification of chemical species by laser initiated non-resonant infrared spectroscopy. A 9.4 micrometer wavelength laser is suggested for producing pulses of laser energy to heat the gas, liquid or aerosol form of a material to be detected. After the unknown mass is heated by a laser pulse, the non-resonant radiation from the heated mass is sampled immediately thereafter. The spectrum is then compared with known chemical specie spectrums to identify the chemical species. Thus, this prior art patent disclosure relies on the absorption coefficient of the gas or vapor, providing a sufficient excitation of the gas or vapor at a specific selected laser frequency. In addition, the use of a pulsed laser system and synchronized sampling of the radiation from the unknown mass requires the use of special timing circuitry.
U.S. Pat. No. 4,490,613 to Brame is directed to an infrared hydrocarbon remote sensor that utilizes a raster scan of a high intensity laser for excitation of hydrocarbon molecules and a receiver that detects re-radiated energy. This disclosure is similar to that of Bernstein, except that in this particular case, continuous or pulsed laser radiation can be used and wavelengths are selected that are more readily absorbed by the hydrocarbon molecules.
U.S. Pat. No. 4,999,498 to Hunt et al is directed to a is remote sensing gas analyzer that uses background radiation present in the area with a spectrometer to detect any gas having characteristic peaks in the infrared region. The radiation from the background passes through the unknown gas and into the analyzer by means of a window and a reflector. A beam splitter and corner reflectors provide an interferogram that is focused by parabolic reflectors for detection by a cooled detector. A single laser is used to determine the position of the scanned corner reflector, The system analyzes the gas absorption when the background is at a higher temperature than the gas. Thus this disclosure utilizes the natural background of the source and interferometer as a receiver.
U.S. Pat. No. 4,864,127 to Brame is directed to an earth surface hydrocarbon gas cloud detection by use of Landsat Data. A satellite receives thermal energy and reflected energy emanating from the earth because of solar energy. The satellite can direct a coherent beam of electromagnetic energy onto its field of view. The signals received by the satellite are sent to a receiving station. The data can reveal hydrocarbon gas presence by converting frequencies to composite wavelengths by combining different bands. The beam of energy directed from the satellite to the earth has a wavelength that is most effective for detecting the hydrocarbon gas cloud. The beam energizes the gas cloud and not the earth. Thus, this particular prior art patent disclosure is based on using natural thermal radiation of the earth and coherent satellite beam radiation to excite a hydrocarbon gas. The specific wavelengths are required for optimum absorption with selection of the wavelength of the satellite laser radiation highly dependent upon the absorption characteristics of the hydrocarbon gas to be detected.
U.S. Pat. No. 4,247,770 to Welch is directed to an aerial, mineral survey method and apparatus using a pulsed laser beam to vaporize surface material. This system's laser generates a high energy beam that vaporizes minerals to be analyzed. The minerals emit an atomic emission spectra characteristic of the material which is analyzed by a spectrometer to determine the makeup of the mineral structures. The infrared energy from the heating is not used to determine the makeup of the vapor by its absorption. Thus, Welch discloses a high power laser to vaporize the samples and means for measuring the atomic spectra of the vaporized samples.
U.S. Pat. No. 4,517,458 to Barringer discloses a system which uses laser energy to cause secondary emission or fluorescence. Unfortunately, the fluorescence cross-section is one or two orders of magnitude less than the absorption cross-section of a given species and therefore requires a much higher power laser compared to a system which relies on absorption.
There is therefore an ongoing need for a remote vapor detection system which does not require extremely high levels of excitation energy, which does not rely for performance on the absorption characteristic of the unknown vapor or gas at a specific wavelength, which does not require complex timing circuitry, which in fact does not rely upon the use of an energy source to heat the vapor or gas to create secondary is emission or fluorescence and which relies upon the use of only a single laser to detect all species within a broad wavelength region, such as 8 to 12 micrometers.